1. Field of the Invention
This invention relates to acoustic sources and particularly to a source having a positively-controlled valve.
2. Discussion of the Related Art
In exploring of the subsurface of the earth, air guns and/or water guns may be used in a body of water to generate an acoustic pulse. The acoustic pulse propagates through the water and into the earth and is reflected from subsurface layers or other objects beneath the water surface and returned where they are detected by a series of sensitive sensors. Each sensor converts the reflected acoustic pulse to either analog or optical signals which may be transmitted to a remote recording and processing device mounted in an attendant ship.
An air gun typically used in seismic exploration generally consists of a housing having a cylindrical valve or shuttle disposed therein to define a firing chamber and a control chamber. The valve separates the control chamber from the firing chamber and closes exhaust ports extending through the housing. The surface area of the valve exposed in the control chamber is greater than the surface area of the opposite end of the valve exposed to the firing chamber, thus a lower pressure in the control chamber offsets a higher pressure in the firing chamber in keeping the shuttle closed. A slight decrease in the control-chamber pressure allowing the pressurized fluid in the firing chamber to open the shuttle and to exit through the now opened exhaust ports. The opening of the shuttle decreases the volume in the control chamber. The decrease in volume may compress any remaining fluid in the control chamber hopefully causing the shuttle valve to rebound and close.
A major disadvantage in using this method to control the closing of the shuttle is the residual fluid compressed in the control chamber by the shuttle may not be adequate to drive the shuttle in the opposite direction to close thus leaving the valve open and allowing water to flow into the firing chamber.
A second disadvantage may be that the shuttle rebounds several times from the closed position before completely closing, resulting in a longer recycling time between firings.